While not limited thereto, the present invention is particularly adapted for use with mining machines of the type wherein separate traction motors are used on opposite sides of the machine. In a mine shuttle car, for example, electrical energy is supplied to the car by means of a cable which is unwound from a reel as the car travels in one direction and reeled back onto the reel as the direction of car movement is reversed. In common types of power supply systems for such mining machines, AC power is supplied to the machine through the aforesaid cable. This AC power is then converted to DC power by means of a phase-controlled SCR bridge converter carried on the machine, the DC power being used to drive the DC traction motors.
In the past, DC traction motors of this type were normally connected in parallel during normal, sustained operation of the machine; but were sometimes connected in series under starting conditions at half speed, the half speed condition being effected by virtue of the fact that the voltage across each individual motor was halved in the series mode. Since the power supplied to the traction motors is the product of voltage times current (i.e., P=EI); and since the voltage across traction motors connected in parallel is the same, power can be increased only by increasing current . Increased current to the traction motors, however, increases the current passing through the aforesaid cable leading to the mining machine, resulting in increased heat generation and the possible requirement for heavier or larger diameter cable. A showing of traction motors connected in parallel can be found, for example, in U.S. Pat. No. 4,042,864 wherein a single full-wave rectifier bridge is used to drive the two motors. In actual practice, however, it has been common to use separate rectifiers or separate SCR AC-DC converters, one for each motor. The use of two power packs, of course, increases costs, complexity and space requirements of the drive system, a factor which is deleterious in mining machines and the like where space and reliability are at a premium.
While heat generation can be reduced by operating the motors in series, series operation is undesirable when, because of mine floor conditions, one of the two drive wheels on the mining machine is slipping. Under these circumstances, increased torque is required on the wheel which is not slipping so that, for a given voltage at the output of the SCR bridge converter, parallel operation is desired such that current through the respective motors can be increased. Thus, while series operation under normal running conditions is desired to reduce current and consequent heating effects, occasions arise where parallel operation is highly desired.
Direct current traction motors for mining machines and other similar vehicles normally employ series field windings wherein the field winding carries the current that flows through the armature. During regeneration in a series motor of this type, voltage generated varies as a function of speed times current. In the past, the maximum attainable voltage was set and the current through the motor was limited. This establishes a maximum speed attainable during regeneration without losing braking action. Beyond that limit, current was reduced in prior art systems to eliminate the possibility of failure to commutate. That is, a regenerative phase clamp was established to eliminate the possibility of too large a negative phase angle, beyond which commutation could not be achieved; and as the phase angle approached but did not reach the regenerative phase clamp value, current was reduced. The difficulty with this is that as current is reduced, speed increases such that the electrical brake becomes ineffective and mechanical braking must be employed.